1. Field of the Invention
The present invention relates generally to the art of drilling large boreholes in earth formations and, more particularly, to the methods of removing the cuttings from the borehole during the drilling operation.
2. Description of the Prior Art
In the drilling of large boreholes in earth formations, a large diameter drill bit is secured to the lower end of a drill stem which is lowered and rotated to cause the bit to operate on the formation being encountered to cut or crush the same. Circulation systems, utilizing drilling fluids such as water or mud, are used to cool and clean the bit and to remove the cuttings produced by the drilling operation from the borehole. Conventional circulation systems pump the drilling fluid downwardly through the drill stem and the fluid returns upwardly through the annulus area between the drill stem and the wall of the hole being drilled. The shortcoming of the conventional circulation systems is that the removal of the cuttings is not efficient in large boreholes because of the large amount of cuttings produced and because the annulus area is much larger than the interior of the drill stem, thereby reducing the return velocity of the drilling fluid and its lifting capacity.
An improved circulation system is described in U.S. Pat. No. 3,208,537 in which an initial small vertical pilot hole extends below the point to which the borehole will later be enlarged and a slant hole is drilled obliquely so that it will intersect the pilot hole near its nether position. The drilling fluid containing the cuttings is then circulated downwardly through the pilot hole and upwardly through the slant hole. An ejector pump is provided in the slant hole to accelerate the upward movement of the drilling fluid therethrough.
The present invention improves on both systems by providing an air lift system for creating a more efficient flow through the drilling fluid return.